termination unit

ABSTRACT

This invention relates to a termination unit comprising an end-section of a cable. The end section of the cable defines a central longitudinal axis and comprising end-parts of N electrical phases, an end-part of a neutral conductor and a surrounding thermally insulation envelope adapted to comprising a cooling fluid. The end-parts of the N electrical phases and the end-part of the neutral conductor each comprising at least one electrical conductor and being arranged in the cable concentrically around a core former with a phase  1  located relatively innermost, and phase N relatively outermost in the cable, phase N being surrounded by the neutral conductor, electrical insulation being arranged between neighbouring electrical phases and between phase N and the neutral conductor, and wherein the end-parts of the neutral conductor and the electrical phases each comprise a contacting surface electrically connected to at least one branch current lead to provide an electrical connection: The contacting surfaces each having a longitudinal extension, and being located sequentially along the longitudinal extension of the end-section of the cable. The branch current leads being individually insulated from said thermally insulation envelope by individual electrical insulators

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT

This invention was made with support under Contract No.DE-FG36-02GO12070 awarded by the United States Department of Energy. TheGovernment has certain rights in this invention.

TECHNICAL FIELD

A power cable installed in an electrical grid needs to connect to thegrid/bus bars through terminations. The present invention relates to atermination unit for electrically terminating a cooled cable system e.g.cryogenically cooled cable system at ambient temperature.

BACKGROUND ART

In general a superconducting (SC) cable must be kept at cryogenictemperatures (0-150 K or −273.25 to −123° C.) in order to function asintended/designed. Usually a cable section connects to other systemcomponents operated at ambient or elevated temperature.

In the following the general term “superconducting cable system” is usedto denote a superconducting and/or hyper-conductive cable (e.g. amulti-phase, such as a three phase, cable) in combination with therelevant thermal insulation envelope.

In order to terminate a SC cable system some basic elements are normallyneeded, namely termination of:

-   -   1. the conductor (current element),    -   2. the electrical insulation (voltage element),    -   3. the thermal insulation (thermal element),    -   4. the cooling means, e.g. fluid cryogen (cooling element), and    -   5. optionally varying diagnostics (diagnostics element).

Items 3), 4) and 5) are elements which are normally not present inconventional cables although 4) has some similarities to cooled bus barsand element 5) is to a certain degree also present in oil insulatedcables where the oil is kept at a certain pressure that is continuouslymonitored. Different optional diagnostics can be implemented, e.g.monitoring of pressure, temperature (internal and/or external), flow,cryogenic fluid level, air humidity, etc.

A tri-axial HTS cable design with three concentric phases surrounded bya concentric neutral conductor (as e.g. described in US 2005173149 A(GOUGE ET AL.) 11-08-2005 and in WO 2006/111170 A (NKT CABLES ULTERA)26-10-2006) have certain advantages over other HTS cable designs.

The advantages over a cold-dielectric co-axial design include:

-   -   1. Reduced use of superconducting material by 34-50% leading to        reduced cost and reduced energy loss.    -   2. Reduced use of cryogenic envelope materials and cold surface        by 30-50% leading to reduced cost and increased energy        efficiency.

Advantages compared to warm-dielectric single-phase cables include:

-   -   1. No external magnetic fields creating disturbances externally        to the cable.    -   2. Improved relation between the electrical properties of        inductance and capacitance leading to longer critical lengths,        improved stability and reduced load-dependant voltage drops.    -   3. Reduced magnetic fields internally in the cable leading to        lower energy losses and improved performance of the        superconducting materials.    -   4. Reduced use of cryogenic envelope materials and cold surface        by a factor 30-50% leading to reduced cost and increased energy        efficiency.    -   5. Reduced number of cryogenic envelopes leading to fewer        welding and fabrication steps, lower fabrication costs and        increased reliability.

Disadvantages compared to the two alternative designs may include thefollowing:

-   -   1. Less well-known dielectric than the warm-dielectric single        phase leading to higher risk in utilisation.    -   2. More complex cable design and termination design than the        co-axial cold dielectric and the warm-dielectric single phase        leading to higher risk in fabrication and in utilisation.    -   3. Inherently/generically imbalanced impedances in phases 1, 2,        and 3.

Advantages of HTS cables over conventional cables with conductors ofcopper or aluminium include normally a higher current carryingcapability, reduced generation and release of heat along the cable,lower electrical loss, and lower weight.

Disadvantages compared to the conventional alternatives include normallythe necessity of a cooling system, continuous thermal loss through thethermal insulation, and increased complexity of accessories such asjoints and terminations.

Termination units for superconducting cable systems are discussed in anumber of prior art documents.

U.S. Pat. No. 6,988,915 B (SEI) Jan. 24, 2006 deals with a terminalstructure of a direct electric current superconducting cable wherein theend portions of the superconducting layers provided over a core linerare exposed in a step-by-step manner from an outer layer to an innerlayer, and outgoing conductors made of a conventional conductivematerial are individually connected with the exposed end portions of therespective superconducting layers. An insulating fixing member supportsthe core and the outgoing conductors.

WO 2005/086306 A (SEI) Sep. 15, 2005 deals with a terminal structure fora multi-phase superconducting cable, wherein an electrically conductivesleeve is disposed around each of the concentrically arrangedsuperconductive layers carrying the electrical phases and electricallyconnected thereto and to leads for extracting each phase at roomtemperature.

U.S. Pat. No. 6,936,771 B (SOUTHWIRE COMPANY) Aug. 30, 2005 deals with atermination unit for connecting a high temperature superconducting (HTS)cable immersed in pressurized liquid nitrogen to high voltage andneutral (shield) external bushings at ambient temperature and pressure.The termination unit comprises a cold housing connected to a warmhousing via a transition duct wherein one or more capillary passagesthrough or parallel to the transition duct allow gas to flow to maintainpressure equilibrium between said cold housing and said warm housing.

US 2005173149 A (GOUGE ET AL.) Aug. 11, 2005 deals with a terminationunit for a superconducting cable comprising three concentricallyarranged superconductive layers (tri-axial). The electrical phaseconductors are terminated to copper tubes. In a preferred embodiment thetubes are concentric and separated by solid insulating tubes. The cableis cooled through liquid coolant streams inside the central tube of thecable and outside of the cable. In a preferred approach, the cold end ofthe termination is conduction cooled from the outside with the liquidcoolant at ground potential. This requires an electrically insulatingmaterial with a high thermal conductivity.

DISCLOSURE OF INVENTION

The objective of the present invention is to benefit from the advantagesof superconducting and hyper-conducting cables by providing a reliableand economic feasibly connection between the high-voltage phaseconductors and neutral conductors in these cables and external equipmentsuch as generators, transformers, breakers, power networks, other cablesor other appliances.

The objective of the invention has been achieved by the invention andembodiments thereof as defined in the claims and as described below.

According to the invention the termination unit comprises an end-sectionof a cable for, one or several branch current leads and a surroundingthermally insulation envelope adapted to comprising a cooling fluid,wherein the branch current leads being individually insulated from saidthermally insulation envelope by individual electrical insulators.

Embodiments of the invention have shown to provide additional benefitsas it will be clear from the following description and claims.

The end section of the cable defines a central longitudinal axis, theend-section comprising end-parts of N electrical phases and an end-partof a neutral conductor, the end-parts of the N electrical phases and theneutral conductor each comprising at least one electrical conductor andbeing arranged in the cable concentrically around a core former withphase 1 located relatively innermost, and phase N relatively outermostin the cable, phase N being surrounded by the neutral conductor.Electrical insulation being arranged between neighbouring electricalphases and between phase N and the neutral conductor. The end-parts ofthe neutral conductor and the electrical phases each comprise acontacting surface for being electrically connected to a current lead(branch current lead). The contacting surface of the neutral conductorand the respective electrical phases is provided by an uncovered part ofthe neutral conductor/electrical phase, i.e. an uncovered part whereelectrical insulation has been removed to expose and provide access tothe contacting surface for providing the connection to the branchcurrent lead. Each of the uncovered parts (which provide the contactingsurfaces) having a longitudinal extension preferably with a minimumlength, and the uncovered parts being located sequentially along thelongitudinal extension of the end-section of the cable.

In the present context the term ‘cable’ is used for a part of the ‘cablesystem’ comprising the electrical conductors and correspondingelectrical insulation between adjacent electrical conductors (andoptionally further layers related thereto). A ‘cable system’ accordingto the invention thus comprises a ‘cable’ in the above sense and athermal insulation surrounding the cable wherein the cable is located.

In an embodiment, the cable is eccentrically located relative to thecentral longitudinal axis of the thermal insulation over at least a partof its longitudinal extension.

The terms innermost/outermost relative to the cable is taken to mean,respectively, closer to/farther away from the cable former.Alternatively, it may be taken to mean, respectively, closer to/fartheraway from the central longitudinal axis of the end-section of the cable.

The numbering of the electrical phases 1, 2, . . . , N is not intendedto imply any particular properties or mutual relationships to the phases(such as e.g. size or angle relationships). In general, the electricalphases may be arranged in any order. In practice, the order providingthe lowest electrical loss may be found by optimization, e.g. byvariation of the phase angles of neighbouring electrical phases, thematerial, number, form, winding angles, etc. of the electricalconductors constituting the electrical phases.

The term ‘uncovered part’ of the cable is in the present context takento mean a length of the cable, where the electrical insulation normallysurrounding an electrical phase (or a neutral conductor) is removed toprovide electrical access to the electrical phase (or neutral conductor)in question.

The term ‘current lead’ and ‘branch current lead’ are usedinterchangeable.

In an embodiment of the termination unit at least one of the electricalconnections between a contacting surface and a neutralconductor/electrical phase comprises an electrical field control systemtotally or partly surrounding the contacting surface and providing anequi-potential volume at the respective phase voltage.

In one embodiment a cooling fluid is adapted to be directed inside saidequi-potential volume. The cooling fluid may in principle be any coolingfluid providing the necessarily cooling.

As it can bee understood this cooling arrangement of the presentinvention provide a highly reliable cooling which stabilise the systemand result in relatively high current ratings. Due to this stable andreliable cooling arrangement several different cooling fluids may beused.

In an embodiment wherein the end-section comprises at least one currentlead base, the termination unit is arranged such that a cooling fluidwill exchange heat with said at least one current lead base above saidcurrent lead and above said contacting surface. Thereby an additionalcooling effect can be obtained.

The current lead base provides transition from current lead to phase orneutral conductor. The current lead base may be provided by theelectrically connection between the (branch) current lead and theneutral conductor/electrical phase.

In the equi-potential volume cooling fluid may preferably heat exchangeswith the current lead base. Additionally in may heat exchanges with oneor more additional electrically conductive elements applied in theelectrical connection between a contacting surface and a branch currentlead. In an embodiment the additional electrically conductive elementscomprises one or more current sleeves arranged to be in good electricalcontact with relevant contacting surface of the conductor(s), one ormore current clamps contacting said current sleeves, and optionallyadditional connecting elements connecting said current lead base withthe clamps.

The current lead base and other electrically conductive elementsenclosed in said equi-potential volume may in an embodiment containprovisions for heat exchange with cooling fluid, said heat exchangerealised by forced flow cooling through and/or passed holes, grooves,fins, profiles but not limited to. The current sleeve and/or currentclamp may have relatively large contacting surface and adapteddimensions may be arranged such that the exact position in longitudinaldirection is not critical but allows a robust longitudinal tolerance.

In one embodiment the equi-potential volume is realised by a number ofinsulator pieces and having a conductive and/or semiconductive innersurface for field controlling reasons.

In an embodiment of the termination unit the equi-potential volumecontains a branch current lead connected to at least one of theelements, a current lead base, a current clamp, and a current sleeve,the branch current lead ultimately being connected to the contactingsurface of the neutral conductor or one of the electrical phases, andwherein at least one of said elements preferably having provision forcold thermal anchoring by forced flow cooling (such as grooves, holes,profiles and flanges) past and/or through the current lead base, currentclamp, current sleeve and/or the interface between either of saidelements, said electrical phase preferably being a conductor selectedfrom a superconductive, a hyper-conductive and a conventional conductor.

In an embodiment the contacting surfaces being located sequentiallyalong the longitudinal extension of the end-section of the cable and arearranged according to a modular concept by ensuring that a thecontacting surface of each of the neutral conductor and electricalphases at least over a part of the lengths of their contacting surfacesare located at a substantially equal radial distance from the centrallongitudinal axis.

This embodiment the termination unit has the additional benefit ofproviding a connection which can be used for different cable types andfor different phase conductors in the same cable type.

The radial distance from the central longitudinal axis of a contactingsurface of the neutral conductor/electrical phase(s) may be regulatedusing a regulating insert, the regulating insert may e.g. be an adaptiveinsert as described below but it may also regulate the radial distancesfrom the central longitudinal cable axis to a contacting surface suchthat these distances remains unequal. The structure of the regulatinginsert may be as described for the adaptive insert below, but withoutthe restriction that the contacting surfaces should be adapted tosubstantially same radial distances from the central longitudinal cableaxis.

In an embodiment the contacting surfaces of the end-section of the cablebeing located with substantially equal longitudinal distance betweenthem. Thereby the termination unit may be even more cost effective toproduce, because the amount of different elements used can be reduced.

Substantial equal longitudinal distance as well as substantially equalradial distance may be obtained by any means. In an embodiment thesubstantially equal radial distances are obtained by radial adaptiveinserts (e.g. fiber-reinforced epoxy plastic such as the material soldunder the trade name G10™) that have substantially identical outerradial dimensions, and where said radial adaptive inserts are applied tothe contacting surface of each phase and neutral equalising theirdimension with respect to longitudinal and specifically radial dimensionin the end-section of the cable.

In an embodiment the substantially equal radial distance from thecentral longitudinal axis of the end-section of the cable of thecontacting surfaces of the neutral conductor and electrical phases issubstantially provided by individual adaptation of inner radialdimensions of such adaptive inserts.

The adaptive inserts may be provided by any suitable materials,preferably comprising at least one of electrically insulating materialand semi-conducting material to prevent an undesired electrically path.In an embodiment the radial adaptive inserts are in the form ofregulating inserts and are made of electrically insulating material,semi-conducting material, or of a combination of such materials.

In an embodiment radial adaptive inserts provide said equal radialdimension of the contacting surfaces wherein at least one of theelements, a current lead base, a current clamp, and a current sleeve areoverlaying the respective adaptive insert to provide the electricalconnection between said respective contacting surfaces and saidrespective branch current leads.

In an embodiment at least one of said adaptive inserts is locatedbetween electrical conductor(s) of the uncovered part (contactingsurface) of the neutral conductor or electrical phase and an underlyinginsulation layer.

The adaptive inserts may have any desired thickness to provide thedesired adapting.

The adaptive inserts provide the advantage of facilitating theconnection of branch current leads to the electrical phases or neutralof the end-section of the cable. They further facilitate a modularbuild-up of the termination unit, including the thermally insulatingenclosure. For example as described below.

The adaptive inserts may further provide mechanical support to theend-section of the cable, and provide a protection against heat damageof the underlying layers of the end-section of the cable duringprocessing (e.g. soldering) and mounting of the end-section of thecable, e.g. in a termination unit.

In an embodiment the termination unit comprising one or severallongitudinal spacers that provides a fixed distance between each of thebranch current leads.

The end-section of the termination unit may for example compriselongitudinal spacers that provide equi-distance between phases andneutral. These longitudinal spacers may additionally provide flowcontrol, flow separation, electric insulation and/or a modularassembly/build system. The end-section may comprise radial adaptiveinserts that provide equalised longitudinal and specifically radialdimension, and the adaptive inserts may for example provide options foradditional dielectric and a modular build concept.

In an embodiment the longitudinal spacer(s) function(s) as flow controlor flow restraint for the cooling fluid guiding at least a part of thecooling fluid through the heat exchanging means near the contactingsurface or the base of the base current lead.

In an embodiment the longitudinal spacers provide substantialequi-distance between each of the branch current leads.

As described above at least one of the neutral conductor or electricalphases may have electrical contact to a current sleeve. The currentsleeve may in principle be of or comprise any kind of electricallyconductive material. Often it is however preferred that the currentsleeve being at least partially of a conventional electricallyconductive material.

In an embodiment, an electrically conductive sleeve is mounted aroundthe cable-end e.g. on top of an adaptive insert. The conductive sleeveis mounted in electrically contact with the contacting surface of theelectrical phase or neutral conductor. The electrically contact maypreferably be provided by a directly contact between the conductivesleeve and the electrical phase or neutral conductor. In an embodimentthe electrical phase or neutral conductor partly overlay the conductivesleeve. In these embodiments superconducting material from the cableitself may constitute at least a part of said sleeve contacting surface.

In a particular embodiment, the at least one conductor of at least oneof the neutral conductor or electrical phases is surrounded by and haselectrical contact to a current sleeve of a conventional electricallyconductive material. In a further embodiment, said current sleeve ismade at least in part by superconducting material and/or hyperconductive material.

The term ‘a conventional electrically conductive material’ is in thepresent context taken to mean a material that is electrically conductivebut has a finite electrical resistance at room temperature as well as atcryogenic temperatures (i.e. a material that is NOT superconductive atthe temperatures in question). Cryogenic temperatures' are taken to meantemperatures below 0° C. (273 K) to which the end-section of the cablesystem is cooled during normal operation, e.g. at or below the boilingtemperature (at a given operating pressure) of the fluid (e.g. liquidN₂) used to cool the end-section of the cable in a termination unit,e.g. temperatures between 0 and 77 K.

In one embodiment, the current sleeve has a sleeve contacting surfacefor being electrically connected to a current lead.

In an embodiment at least a part of the current sleeve provide theelectrically connection between said contacting surfaces and saidrespective branch current leads. In this embodiment the current sleevemay have a sleeve contacting surface electrically connected to a branchcurrent lead.

In a particular embodiment, wherein the termination unit comprises twoor more current sleeves in electrically connection with neutralconductor/electrical phase(s) and with respective sleeve contactingsurfaces, these sleeve contacting surfaces are located at asubstantially equal radial distance from the central longitudinal axisof the end-section of the cable.

In an embodiment wherein two or more of the neutral conductor orelectrical phases have electrical contact to respective current sleeves,the dimensions of said current sleeves may preferably be substantiallyidentical.

In an embodiment a substantially equal radial distance from the centrallongitudinal axis of the sleeve contacting surface of the neutralconductor and electrical phases is provided, preferably by individualadaptation of radial dimensions of said current sleeves. For example thecurrent sleeves may have different inner dimensions and essential equalouter dimensions.

In an embodiment, the sleeve(s) comprises a sloping section forconnecting to the tape/wire conductors of the superconducting cable,e.g. through soldering.

In a further embodiment the soldering section is manufactured withterraces adapted in length and height to the relevant HTS tapes/wiresand optional shunting conventional conductor tapes.

The radial regulation of said contacting surface/sleeve contactingsurface of each electrical phase or neutral to achieve a constant radialdistance of said surface from the longitudinal axis of the end-sectionof the cable can be achieved in any appropriate way by proper adaptationof the materials and the construction underlying the contacting face.

In a particular embodiment, the radial extension of at least one of theelectrically insulating and electrically conducting materialssurrounding or underlying the individual contacting surfaces of theneutral conductor or electrical phases is individually adapted toprovide the individual contacting surfaces and/or the individual sleevecontacting surfaces with a constant radial distance from thelongitudinal axis.

In a particular embodiment, an adaptive insert of an electricallyinsulating material, semi-conducting material, or of a combination ofsuch materials, is inserted between the end parts of the neutralconductor or electrical phase and the neighbouring, underlyingelectrical phase at the location of the contacting surface of theneutral conductor or electrical phase in question.

In a particular embodiment, at least two electrical phases or neutral,such as a majority or all electrical phases or neutral, comprise sleeveshaving substantially identical outer and inner radial dimensions.Preferably all electrical phases and neutral being provided with acurrent sleeve comprising substantially identical dimensions.

In an embodiment the termination unit comprises at least one currentclamp made at least in part from a conventional electrically conductivematerial, such as, but not limited to Cu. The current clamp beingelectrically connected to a current lead, and being clamped to saidcontacting surface and/or to said sleeve contacting surface. The currentclamp may preferably be shorter than the axial length of the contactingsurface and/or shorter than the sleeve contacting surface. Thereby thecontacting surface and/or the sleeve contacting surface presenting abroad electrical contact (in longitudinal terms) to said current clamp,allowing a large tolerance of the movement of the current clamp/currentlead in a longitudinal direction of the end-section of the cable (e.g.during mounting and heating/cooling of the cable system).

In an embodiment at least one of said current clamps comprises throughgoing holes for additional cooling.

In general it is normally desired that at least one of the electricalphases and neutral conductor comprises superconductive orhyper-conductive material.

In an embodiment at least one of the electrical connections comprises anelectrical field-control system totally or partly surrounding thecontacting surface and at least a part of said branch current lead. Bythis electrical field-control system a very reliable and stable systemcan be obtained.

The electrical field-control system may comprises a dielectrics, such asthe dielectric sold under the trade name ULTEM™ or fiber-reinforcedepoxy plastic such as the dielectric sold under the trade name G10. Inprinciple any other suitably dielectric may be used.

In an embodiment the electrical field-control system further comprises afield smoothening material e.g. a metal- or semi-conducting material.The field smoothening material preferably constitutes a layer of theelectrical field-control system, more preferably an inner layer of theelectrical field-control system.

In an embodiment the electrical field-control system provides atransition joint to one of said branch current lead and said branchelectrical insulation insulating said branch current lead. Thetransition joint may preferably be made between surfaces formingmatching zig-zag paths or meander paths to increase the creep length orsaid joint having spherical surfaces to allow for angular adjustment ofthe branch insulation system, preferably said transitional joint issealed by a gasket.

In an embodiment at least one of said branch current leads comprising acurrent lead section, and this current lead section (which may be thewhole or a part of the branch current lead) comprising a cold endconnected to a neutral conductor or an electrical phase, an oppositeambient temperature end and an intermediate thermal anchor section Theintermediate thermal anchor section provides the a thermal gradient fromcold to ambient temperature.

In an embodiment at least one of said branch current leads comprising acurrent lead section comprising thermal anchor section that provides theoption of a thermal gradient from cold to lower temperatures.

The termination unit may in an embodiment comprise at least one of asleeve, an insert and a clamp wherein said sleeve(s), insert(s) and/orclamp(s) has/have each the form of a cuff.

In one embodiment the group consisting of the sleeves, inserts andclamps comprise two, three or more parts for facilitating easy mounting.

For controlling the electrical field the edges and/or surfaces oflongitudinal spacers, adaptive inserts, sleeves, clamps, current leadbases may preferably be rounded.

In an embodiment the termination unit comprises at least one insert(adaptive insert) made of material having a specific electricalresistivity above about 10̂6 Ω·m²/m over the operating temperature range.

In an embodiment the termination unit comprises at least one insert(adaptive insert) comprising an epoxy material, such as a fibrereinforced epoxy resin, such as a thermosetting industrial laminateconsisting of a continuous filament glass cloth material with an epoxyresin binder.

In an embodiment terminal unit comprising a rigid cylinder or tubelocated in an opening at the centre of the core former of theend-section of the cable, for providing mechanical support for the endsection of the cable.

In an embodiment the termination unit is adapted for providing andelectrically terminating a cryogenically cooled multi-phase cable systemto termination current leads at ambient temperature. The cableend-section is an end section of a multi-phase cable. The end-section ofthe multi-phase cable is surrounded by a thermally insulating envelopefor enabling the cooling to—and the maintaining of at least a part ofthe end-section of the cable at a temperature below ambient, wherein thethermally insulating envelope comprises a plurality of branches in theform of branch current lead for branching off from the end-section ofthe cable electrical phases and the neutral conductor to respectivecurrent leads. The branches being arranged sequentially along thelongitudinally extending end-section of the cable at locations matchingthe location of the contacting surfaces of the corresponding end-partsof the neutral conductor and electrical phases in question. The firstbranch being for branching off the neutral conductor, the second branchfor the N^(th) electrical phase, the third branch for the (N−1)^(th)electrical phase, etc., and finally the (N+1)^(th) branch for branchingoff the N^(th) electrical phase.

The thermally insulating envelope may preferably have an innercylindrical surface, the end-section of the cable being located alongthe surface and the points of contact between said surface and saidcable defining a substantially straight contacting line, to thereby makea simple and reliable construction.

At least one, preferably all of the branches may in an embodiment besubstantially perpendicular to the longitudinal axis of the end-sectionof the cable.

In an embodiment the thermally insulating envelope enclosing saidend-section of said cable is modularly constructed, each modulecomprising a part for enclosing a length of the end-section of the cableand at least one branch for branching off the neutral conductor or anelectrical phase.

For increased cooling the thermally insulating envelope may in anembodiment comprise two separate fluid coolant flow paths.

At least a part of the fluid coolant in the thermally insulatingenvelope may be restricted to a flow path that is unique for thetermination unit and does not pass through the rest of thesuperconducting cable.

In an embodiment the termination unit comprises a field-control systemin contact with an electrically insulating material located around theend-section of the cable at branches of the thermally insulatingenvelope, the field-control system preferably defining an equi-potentialvolume at phase voltage and controlling the path of the electric fieldsin the vicinity of the branch from phase voltage to a ground potentialof the termination housing.

Such field-control system may for example comprise a material selectedfrom the group of materials comprising the dielectric sold under thetrade name Ultem™, the dielectric sold under the trade G-10™, FRP,Polyethylene, Polypropylene, Nylon and combinations thereof.

In an embodiment, the contacting surface of a given electrical phase orneutral has a length L_(ucp) allowing electrical connection to a currentlead for terminating the phase or neutral. In an embodiment, L_(ucp) isadapted to allow a modular build-up of a thermally insulating envelopesurrounding a length of the end-section of the cable and comprising atleast one branch for housing a current lead. In practice, L_(ucp) islarger than 0.1 m, such as 0.2-0.3 m.

An embodiment of the invention comprises a cooling arrangement where acooling fluid exchanges heat in the region of the connecting surfaces ofthe end section. In a particular embodiment, the cooling fluid passesthrough a connecting block connecting the base of a current lead to acurrent sleeve. In a further embodiment, the cooling fluid passes closeto the contacting surface, in the contacting surface or inside thecurrent sleeve. In yet another embodiment, the cooling fluid passesclose to the base of the current lead or through the base of the currentlead, in such a way that heat transported through the current lead orgenerated by electrical currents in the current lead is transferred tothe cooling fluid and removed from the contacting surface withoutsignificantly increasing the temperature of the superconducting cable.In a preferred embodiment, heat exchanges between the base of thecurrent lead, the contacting surface and the cooling fluid through solidmaterials with high thermal conductivity, such as copper, silver, gold,aluminium. In one embodiment, flow control tubes guide a portion of orall of the cooling fluid in the cable system towards and through saidcooling arrangement.

In one embodiment of the invention, an electrical field-control systemcomprising one or more field-control elements of a field control system,forms an equi-potential volume. In a particular embodiment, said volumecontains the above said cooling arrangement. In a preferred embodiment,the field-control system is applied to or in intimate contact with anouter solid electrical insulation material, such as polymeric orcellulose or ceramic or glass or composite based electrical insulation.

In a further embodiment, said field-control system is provided in theform of a metal- or semi-conducting surface connected to the phasepotential of the enclosed contacting surface. In one embodiment, saidelectrical field-control system considers the axial phase-to-phaseelectric fields and the radial phase-to-ground electric fields byproviding sufficiently long creep lengths and sufficiently thickelectrically insulating material. In a preferred embodiment, a fluidsurrounds the electrical field-control system and outer electricallyinsulating material. In one embodiment, the electric field-controlsystem provides a transition joint to a branch current lead and branchelectrical insulation.

In a preferred embodiment, the electrical field-control system and thebranch insulation system is provided in a single part. In a furtherpreferred embodiment, the electrical field-control system and the branchinsulation system is provided in two or more parts with at least onejoint, said joint surfaces forming matching zig-zag paths or meanderpaths to increase the creep length or said joint having sphericalsurfaces to allow for angular adjustment of the branch insulationsystem. In one embodiment, said joint is sealed by a gasket made frommaterials such as PE, Polypropylene, PTFE, Tyvek™, Nomex™, Teflon™ orGore-Tex™ in order to increase the electrical creep strength.

The covered part of a phase or screen may be covered by electricalinsulation and/or semiconducting material. This can include alength-section of the conductive sleeve, for example by windingelectrically insulating tapes over a part of the conductive sleeve. Inone embodiment, electrically insulating tapes are wound over the regionof the sleeve where superconducting tapes are joined to the sleeve.

The number of phase conductors N may in principle be as desired. N, mayfor example be 1, 2, 3, 4 or more, preferably N is 1 or 3.

The concentric arrangement of a single (N=1) electrical phase andneutral in a cable is also termed a coaxial cable.

The concentric arrangement of a multitude (N) of electrical phases andneutral in a cable is also termed a multi-coaxial cable.

In an embodiment, the contacting surfaces of a multi-coaxial cable isrealised in a longitudinal and sequential way characterized in that theneutral-to-phase contact distance and phase-to-phase contact distancesare approximately the same.

The term ‘ambient temperature’ is in the present context taken to meanthe temperature at the location where the electrical phases (or neutral)are terminated. Such temperature can e.g. be in the range from −50° C.to +85° C., e.g. between −30° C. and +50° C. or between −10° C. and +30°C.

Further objectives of the invention are achieved by the embodimentsdefined in the dependent claims and in the detailed description of theinvention. It should be emphasized that the term “comprises/comprising”when used in this specification is taken to specify the presence ofstated features, integers, steps or components but does not preclude thepresence or addition of one or more other stated features, integers,steps, components or groups thereof.

BRIEF DESCRIPTION OF DRAWINGS

The invention will be explained more fully below in connection with apreferred embodiment and with reference to the drawings in which:

FIG. 1 shows a cross section of a triax cable.

FIG. 2 shows a sketch of a 3-phase cable termination.

FIG. 3 shows a sketch of a termination built by using a modularapproach.

FIG. 4 shows a single-phase coaxial termination built using a modularapproach.

FIG. 5 shows an embodiment of an ambient thermal anchor.

FIG. 6 shows different examples of accessories that can be applied on atermination module.

FIG. 7 shows the different elements of an embodiment of a current lead.

FIG. 8 a shows several schematic examples of how and where to locate theheat exchanger providing the cooling means of conventional tosuperconducting transition.

FIG. 8 b shows a detailed embodiment of how to connect the current leadwith the conductor including location of a heat exchanger providing athermal anchor.

FIG. 9 a shows a sketch of the equi-potential volume at phase potentialthat is defined by a conductive element inside a dielectric elementarranged around a connecting region between the phase conductor andcurrent lead.

FIG. 9 b shows a detailed embodiment of the equi-potential volume atphase potential that is defined by a conductive element inside adielectric element arranged around a connecting region between the phaseconductor and current lead.

FIG. 10 shows one embodiment of cooling fluid injection or return.

FIG. 11 a shows one cooling configuration with separate cooling oftermination and cable at the supply end.

FIG. 11 b shows one cooling configuration with separate cooling oftermination and cable at the opposite end.

FIG. 12 shows an optional transition between cryostat and termination.

FIG. 13 shows an optional mechanical fixation of the cable with respectto the termination.

FIG. 14 shows an embodiment of a mechanical rigidizer enabling amechanical robust construction.

FIG. 15 shows in detail one way of terminating the HTS and Cutapes/wires.

FIG. 16 shows a concept of the adaptive insert that equalises the radialdimension of the different conductor phases and the neutral. The conceptworks as well for different cable dimensions within reasonably similarratings.

The figures are schematic and simplified for clarity, and show detailsthat are essential to the understanding of the invention, while otherdetails are left out. Throughout, the same reference numerals are usedfor identical or corresponding parts in the sense that a feature ‘102’in FIG. 1 is referred to as ‘202’ in FIG. 2, i.e. the Fig. numberprecedes the ‘through-going’ reference numeral.

Further scope of applicability of the present invention will becomeapparent from the detailed description given hereinafter. However, itshould be understood that the detailed description and specificexamples, while indicating preferred embodiments of the invention, aregiven by way of illustration only, since various changes andmodifications within the spirit and scope of the invention will becomeapparent to those skilled in the art from this detailed description.

EXAMPLES OF THE INVENTION

Embodiments of this invention sets out to enable an easy andstandardised connection to the a superconducting cable (ac or dc)including power in and out of the cable, controlled individual coolingof the cable, the cable end section, the current leads and the inside ofthe termination enclosure. Further, compensation of cable andtermination thermal contraction, easy assembly as well as effective costand effective production of the termination unit(s)/module(s) may befacilitated by the invention.

FIG. 1 is a cross-sectional view of a coaxially arranged multi-phase(here triax) cable 100 comprising N=3 phases and a neutral (screen),eccentrically located in a thermally insulating envelope 102C. The threephases (referred to as phase 1, 2, 3 or 100R, 100S, 100T) and theneutral are placed concentrically around each other, in this case withthe innermost located is phase 1, the one located in the middle is phase2 and the outermost located is phase 3 surrounded by the neutral andfinally the thermally insulation envelope, 102C. The cable 100 isimpregnated with a cooling fluid, 101, e.g. liquid Nitrogen, in that thespace between the inner wall of the thermally insulating envelope andthe central opening of the cable (i.e. inner volume of the centralformer 103 which here supports phase 1) is filled with a cooling fluid101. Between each phase and between phase 3 and the neutral, a cryogenicdielectric is applied. This invention applies to both AC and DC cables.The cable is e.g. applicable for DC transmission, if the three phasesare substituted with minus (phase 1), zero (phase 2), plus (phase 3) andneutral conductor.

In order to terminate this rather complex structure, embodiments of thisinvention are designed to address and solve a number of issues relatingto terminating the electrically conducting as well as the thermallyinsulating layers. One embodiment of a termination unit is shown in FIG.2, where all phases (here 3) and the neutral are terminated in onecomplete construction. The FIG. 2 illustrates a completevacuum-insulated termination unit (could alternatively or in addition befoam insulated) housing 202 with an optional thermal compensation 221between the sequentially branched-off phases or neutral. Further, allthe main body branches (i.e. the parts of the thermally insulatingenvelope branching off from the longitudinal direction of theend-section of the cable) are (here) similar and at ground potential andat 90° to the longitudinal direction of the end-section of the cable.Alternatively, the stand-offs could be different (e.g. between neutraland electrical phases), e.g. individually adapted and not necessarilyperpendicular to the longitudinal direction of the end-section of thecable. On top of all 4 branches, an intermediate section 211 is attachedwith the function of controlling the fluid level as well as ensuring athermal anchor at ambient or elevated temperature for the electricalinsulator, 212. Above the thermal anchor section 211, a standardelectrical insulator, 212, is applied. This is (here) omitted on theneutral. Above the electrical insulator 212, the current lead isterminated in a top bolt, 214, at ambient temperature and at gridvoltage inside optional warm cover 213. Said current lead can comprise aflexible section (cf. e.g. 714 on FIG. 7). Between different parts ofthe branched-off electrical phases or neutral, sealing flanges 222 aremounted (cf. e.g. between a branch of the termination housing 202 and athermal anchor section 211, between a thermal anchor section 211 and awarm cover-section 213 and between a warm cover-section 213 and a topbolt 214. The thermally insulating envelope 202D connected to a cryostat202C is shown to be modularly constructed and to comprise 4 identicalT-sections. At both ends, special parts are needed, at the interface tothe cable an adapter-piece 225 and at the other end a closing end-piecefor terminating the thermally insulating envelope 202D in thelongitudinal direction of the end-section of the cable, both piecesoptionally housing in- and/or out-lets for cooling fluid and/ormonitoring sensors 281. Further, an optionally movable support structure298 is shown comprising wheels or rollers for allowing the terminationunit to move and thereby adapt to thermal expansion or shrinkage of thecable. As it can be seen, due to this T-section structure the branchcurrent leads in the branches being individually insulated from thethermally insulation envelope by individual electrical insulators. Thetermination housing 202 and the individually insulatings which insulatethe branches constitute here the total thermal envelope 202D.

In this embodiment the T-housing has a transition piece, 225, FIG. 2,interfacing the termination unit with the cable thermal envelope. Thistransition section can be individually evacuated in the field, so thetermination unit and the cryostat can be prefabricated andpre-evacuated. The transition piece may be foam insulated as well asvacuum insulated.

Other embodiments of a termination unit are shown in FIG. 3 and FIG. 4where the modular approach is featured. In this case the idea is tobuild a complete cable termination unit by connecting a number of (e.g.2 or 3 or 4 or more) similar modules thus improving productionefficiency. Each modular envelope element 302, 402 is supplied with asealing flange 322, 422 at each end for being coupled to anotherenvelope element. Each modular envelope element has one or more in-and/or out-lets for cooling fluid and/or monitoring sensors 381, 481.

FIG. 3 shows vacuum insulated modular termination housings, 302. In thiscase, the branches for the electrical phases (two rightmost) areidentical but different from that of the neutral (leftmost). Optionalthermal contraction compensation elements 321 are shown at variouslocations on termination housing 302. Each branch is coupled to afluid-level control section comprising a thermal anchor at ambienttemperature 311 via a sealing flange 322, which again (also via asealing flange 322) is coupled to an electrical insulator section 312.The electrical insulator section 312 is optional and included dependingon function (neutral or phase) and depending on voltage rating. Abovethe electrical insulator, at high potential, is the current terminationcover 313, which is coupled to a top bolt 314 via a sealing flange 322.

FIG. 4 shows an embodiment of the modular approach where the branchesare minimised, i.e. where the cold to ambient transition is integratedinto the voltage transition, insulator 412. Further, it illustrates thatthe different sections (fluid level and thermal anchor section 411,electrical insulator section 412 and current termination cover-section413) can be connected in different ways.

FIG. 5 shows the thermal anchor section 511 at ambient temperature andat ground potential implemented by a heater 585. Alternatively, thisembodiment is realised by any combination of a controlled heater, a heatradiation source, an infra-red lamp, a fan, cooling ribs or similar.

FIG. 6 demonstrates the application of thermal sensors 684, fluid levelgauge 682, pressure gauge 683 as well as vacuum gauge 681 for the vacuuminsulation. Similarly a flow detector could be applied close to or in aneutral branch. 601 is the cooling fluid. The end-section of the cableis denoted by reference numeral 600. The current lead 614 is shown to beconnected to a current sleeve 616 having electrical contact to thecontacting surface of electrical phase (or neutral) via a current clamp615 mounted around and having electrical contact to the current sleeve616.

FIG. 7 shows details of a flexible branch current lead. The element isbuilt up by several parts and sections in order to ease the installationof the branch current lead. This, however, does not prevent the branchcurrent lead from being implemented in one complete assembled piece. Inthis case, however, it would be necessary to build up the terminationhousing around the current and voltage element in the field instead of,like in this case, insert and connecting the different parts andsections inside the completed termination housing and insulation. InFIG. 7 a, the current lead consists of a stiff part 714C, a flexiblepart 714B for compensating the thermal contraction, a top bolt part 714Athat constitute a standard interface with the grid, a hermetic top boltpass-through 722 enabling a pressure difference between internal andexternal, a current lead that interfaces with a current clamp 715.Easily accessible connections 714D between the flexible part 714B andthe stiff parts 714A and 714C of the current lead are shown and may beimplemented by a removable lid in 713, cf. FIG. 7 b.

Individually, these parts can be joined optionally with bolts orsoldering/brazing/welding or glueing. The seat surface area is maximisedto obtain a low resistant connection. In the said embodiment the currentclamps 715 contact to the current sleeve 716 that is the actual currenttermination of a phase conductor. In this case the current sleeve andthe phase conductor are connected by soldering. In this embodiment theconnecting between the current sleeve and the clamp as well as betweenthe clamp and the lead are provided by bolting the elements together. Inanother embodiment the same parts can be solder connected, either of theparts can be omitted or any combination of soldering and boltingtogether can be applied in order to minimise connection resistance andprovide ease of assembly. In this example all parts are made of Cu butcould be any other material or compound of material with an appropriateelectrical resistance (including at cryogenic temperatures).

FIG. 8 a shows a module forming a current branch element of atermination unit comprising the end-section of the cable 800, a sleeve816, a clamp 815, a current lead seat assembly (current leas base) 814,814E, and alternative means for heat exchange in the form of coolingchannels, 818. The electrical phase conductor and the sleeve are solderconnected. The sleeves, the clamps and the current leads can be similarfor all the phases.

In the ground and/or neutral connection these parts may be similar butwill typically be of a more lean design in order to minimise heat leakfrom the ambient to the cryogenic section. Relating to all theelectrically conducting parts each of them can be independently omitted,connected by soldering, brazing, welding, melted, glued, exploded or anyother chemi-physical means or connected by bolting, clamping or anyother mechanical means. In order to improve electrical and/or mechanicalcontact the parts can be surface treated to obtain a particular physicalappearance and/or coated to inhibit corrosion or chemisorbedcontamination.

In FIG. 8 b, an example is given on the current sleeve being adapted tothe superconducting tapes minimising the mechanical strain as well asoptimising the electrical contact by having a smooth conical interfacebetween conductor 800 and current sleeve 816 (cf. left part of thesleeve 816). In this example, the connection is made by soldering. Thesleeve is positioned and centred around the underlying (phase or former)with a regulating insert 817 that could be made of fibre-reinforcedpolymer (FRP) or other electrically insulating material operational atlow cryogenic temperature. In this embodiment, the regulating insert mayalso constitute an adaptive insert and serve the purpose of presentingan equal radial dimension to the current sleeve 816 so that the samesleeve unit can be used for all phases as well as for the groundconnection (neutral connection). Often, however, the ground may bedifferently designed. The clamp has longitudinal holes 818 for passingcooling fluid 801 through. In this way the clamps serve as heatexchanger and ultimately as efficient thermal anchor at cryogenictemperatures. Alternatively, the holes and thereby the thermal anchorcould be located in the current lead seat 814E, in the sleeve 816 or anycombination of locations (cf. FIG. 8 a). As it is indicated theinterface between current lead seat 814E and top part 815A of currentclamp may be zig-zag shaped.

FIG. 9 a shows a simple field-control system comprising an electricalfield-control element 956, e.g. in the form of a metal orsemi-conducting surface. The field-control element is galvanicallyconnected 957 to the phase potential of the cable conductor 900R andthereby defines an equi-potential volume containing cooling fluid 901and the cooling means 918. Coolant flow guiding tubes, 950, guides apart of the coolant 901 through the equi-potential volume.

In FIG. 9 b, the field-control system is provided in the form of anelectrical field-control element (956, 951, 952) built up fromindividual parts in order to ease the installation and in order toseparate the functions of each element. Apart from the genericdielectric of the cable, 957, the dielectric of the branch consists ofthe regulating insert 917 (which can be electrically insulating) betweenconductor 900 and sleeve 916, a prefabricated horizontal dielectric 950made from e.g. G10-type fiber-reinforced epoxy plastic, forming acompound dielectric together with the cooling fluid and guiding thecoolant flow. In this illustration the element 950 also constitutes alongitudinal spacer/stand-off between two phases or phase to neutral.Thereby substantially equal longitudinal distance between the contactingsurfaces can be obtained. A prefabricated vertical dielectric 952, madefrom e.g. G10, is located between the vertical branch wall of thetermination housing, 902, and the current lead, 914. The element 952 hasfield-control system comprising one or more elements such as metalcoating on the inside/high voltage side, 956, extended creep distancesin interface, 958, between 952 (G10) and 951 (ULTEM™). There is filleron the inside 954, and on the outside 953 of the vertical dielectric 952as well as a centring and fixing means 955. Between the verticaldielectric 952, the outer 953 and inner 954 filler is cooling fluid, thefluid also optionally impregnating the dielectric and/or the filler. Thehorizontal dielectric 951 (ULTEM™) has a double function as facilitatingthe transition from phase to ground (housing) of one phase (e.g. 900R)at the same time being part of the compound dielectric (dielectric 950,951 and cooling fluid 901) and field-control element(s) (956) of thetransition between one phase (900R) and the succeeding phase (900S) ofthe conductor 900. Further, the element 951 provides increased creepdistance at the horizontal opening, 959. There are field-controlelement(s) integrated in the adaptive insert, 917E (edge shape) and thecurrent sleeve, 916E (edge shape).

The individual parts of the voltage branch element in FIG. 9 b are madeeither of the same dielectric material or a combination of differentdielectric material. The voltage branching arrangement (951+952+956) andthe current branching arrangement (916+915+914) are integrated andtogether provide an equi-potential volume containing the means for heatexchange, 918, between the cooling fluid, the contacting surfaces andthe base of the current lead. The parts support each other mechanically.

In FIGS. 10, 11 a and 11 b two embodiments of cooling fluid in/out areshown. In FIG. 10, the cooling fluid is injected with the option ofadjusting the central flow in relation to the annular flow. With thecontrolling valves 1086 it is possible to direct the cooling fluid tothe centre, 101 c (FIG. 1) of the conductor and/or to the annulus, 101 a(FIG. 1) between conductor and thermal envelope. Further, the coolingfluid is used for cooling the cable as well as cooling the terminationpart.

In FIG. 11 a, one embodiment is implemented where the terminationprovides a counter flow configuration for the cable. The warm coolingfluid from the cable is extracted through 1187 g near the neutral. Closeby, cold cooling fluid is injected through 1187 f, in a separate annularflow through the termination. The central flow is injected into thetermination end, 1186 f. In the case that the valve 1186 f is closed,the cable and termination would operate with stagnant cooling fluid inthe centre. The advantage of this supply and return configuration isthat the load dissipated by the termination is not carried through thecable thus providing an increased thermal window for the cable.

In FIG. 11 b, said counter flow configuration of the cable isimplemented in the opposite end of the cable system. The cooling fluidis injected near the neutral current lead, 1187 f, then splitting intothe cable annular flow and a separate annular flow through thetermination. The central flow of the cable and the annular flow of saidtermination are extracted from the termination end through valve 1186 fand 1186 g. In the case where the valve 1186 f is closed, the cable andtermination would operate with stagnant cooling fluid in the centre. Theadvantage of this supply and return configuration is that the loaddissipated by the termination is not carried through the cable thusproviding an increased thermal window for the cable. An optional cooler1188 is indicated for regenerating the cooling fluid.

FIG. 12 shows an optional adaptor or transition piece, 1225, connectingthe termination, 1202, to the cryostat, 1202C. The adaptor, 1225,includes optional thermal contraction compensation, 1221D and vacuumvalve and/or gauging 1281. In this embodiment the transition piece 1225is vacuum insulated, however, it can also be foam insulated or acombination of foam and vacuum insulated.

FIG. 13 illustrates internal clamps, 1318 that mechanically fixes thecable with respect to the termination 1302. In this embodiment themechanical fixation is located in the cryostat to termination adaptor,1325, however, it could easily be located also in the neutral section ofthe termination 1302 or another section at ground potential where themechanical fixation does not compromise the electrical insulation.Alternatively the fixation could be manufactured in a dielectricmaterial and integrated with the other electrical insulation components.

FIG. 14 illustrates the internal part of one end section of a phase 1400R, S or T or neutral where the former 1400 f is mechanically supportedby a rigidizer 1419.

The rigidizer, 1419 can be cylindrically symmetrical and may be hollowfor enabling flow of cooling fluid or solid thus preventing any flowthrough the centre of the cable end section.

FIGS. 15A and 15B show a single phase or neutral where the HTStape/wires 1500 and/or the Cu tapes/wires, 1500 are joined to thesleeve, 1516. In 15B the sleeve, 1516 is showed in detail illustratingthe terrace-like connecting area that may be adapted to whether it is aconnection with HTS tapes/wires or with Cu tapes/wires. In the presentembodiment an example is given with overlap region being 2 cm for Cu and5 cm for HTS tapes/wires. The step height of the terraces may optionallybe adapted to the thickness of the used HTS or Cu tapes/wires. Further,the adapting insert, 1517 is shown to be located around either anunderlying electrically insulating neighbour phase, alternatively aroundthe central former.

FIG. 16 illustrate the concept of having a prefabricated regulating e.g.adaptive inserts, 1617 that can be adapted to the radius of differentphases or neutral conductors that may be exposed. The adaptive insertmay be manufactured in e.g. G10. The adaptive insert 1617 present to theoverlaying current sleeve, 1616, a uniform dimension such that onesingle sleeve shape fits all the phases and neutral as well as can beused in completely different cable terminations, as long as the highvoltage part and current ratings are within reasonably similar ranges.Alternatively the current sleeve, 1616, may be manufactured as anadaptive piece, thus presenting to the current clamp, 1615, a uniformdimension. Further, the current sleeve 1616 has a surface mating withthe current clamp, 1615 that is longer than the current clamp enabling alimited range of longitudinal freedom for positioning the current clampand thereby the current lead.

Some preferred embodiments have been shown in the foregoing, but itshould be stressed that the invention is not limited to these, but maybe embodied in other ways within the subject-matter defined in thefollowing claims. The examples are shown for a 3-phase cable (triax),but might easily be modified to any other number of electrical phases.

1. A termination unit comprising an end-section of a cable, the endsection of the cable defining a central longitudinal axis and comprisingend-parts of N electrical phases, an end-part of a neutral conductor anda surrounding thermally insulation envelope adapted to comprising acooling fluid, the end-parts of the N electrical phases and the end-partof the neutral conductor each comprising at least one electricalconductor and being arranged in the cable concentrically around a coreformer with a phase 1 located relatively innermost, and phase Nrelatively outermost in the cable, phase N being surrounded by theneutral conductor, electrical insulation being arranged betweenneighbouring electrical phases and between phase N and the neutralconductor, and wherein the end-parts of the neutral conductor and theelectrical phases each comprise a contacting surface electricallyconnected to at least one branch current lead to provide an electricalconnection, the contacting surfaces each having a longitudinalextension, and being located sequentially along the longitudinalextension of the end-section of the cable, said branch current leadsbeing individually insulated from said thermally insulation envelope byindividual electrical insulators.
 2. A termination unit according toclaim 1 wherein at least one of said electrical connections comprises anelectrical field control system totally or partly surrounding thecontacting surface and providing an equi-potential volume at therespective phase voltage.
 3. A termination unit according to any one ofclaims 1-2 wherein a cooling fluid is adapted to be directed inside saidequi-potential volume.
 4. A termination unit according to any one ofclaims 1-3 wherein said end-section comprises at least one current leadbase, said termination unit being arranged such that a cooling fluidwill exchange heat with said at least one, current lead base above saidcurrent lead and above said contacting surface.
 5. A termination unitaccording to any one of claims 2-4 wherein said equi-potential volumecontains a branch current lead connected to at least one of theelements, a current lead base, a current clamp, and a current sleeve,the branch current lead ultimately being connected to the contactingsurface of the neutral conductor or one of the electrical phases, andwherein at least one of said elements preferably having provision forcold thermal anchoring by forced flow cooling (such as grooves, holes,profiles and flanges) past and/or through the current lead base, currentclamp, current sleeve and/or the interface between either of saidelements, said electrical phase preferably being a conductor selectedfrom a superconductive, a hyper-conductive and a conventional conductor.6. A termination unit according to any one of claims 1-5 wherein thecontacting surfaces being located sequentially along the longitudinalextension of the end-section of the cable and are arranged according toa modular concept by ensuring that the contacting surface of each of theneutral conductor and electrical phases at least over a part of thelengths of their contacting surfaces are located at a substantiallyequal radial distance from the central longitudinal axis
 7. Atermination unit according to any one of claims 1-6 wherein thecontacting surfaces of the end-section of the cable being located withsubstantially equal longitudinal distance between them.
 8. A terminationunit according to any one of claims 6-7 wherein said substantially equalradial distances are obtained by radial adaptive inserts (e.g.fiber-reinforced epoxy plastic such as G10) that have substantiallyidentical outer radial dimensions, and where said radial adaptiveinserts are applied to the contacting surface of each phase and neutralequalising their dimension with respect to longitudinal and specificallyradial dimension in the end-section of the cable.
 9. A termination unitaccording to claim 8 wherein the substantially equal radial distancefrom the central longitudinal axis of the end-section of the cable ofthe contacting surfaces of the neutral conductor and electrical phasesis substantially provided by individual adaptation of inner radialdimensions of said adaptive inserts.
 10. A termination unit according toany one of claims 8-9 wherein said radial adaptive inserts are in theform of regulating inserts and are made of electrically insulatingmaterial, semi-conducting material, or of a combination of suchmaterials.
 11. A termination unit according to any one of claims 8-10where said radial adaptive inserts provide said equal radial dimensionof the contacting surfaces wherein at least one of the elements, acurrent lead base, a current clamp, and a current sleeve are overlayingthe respective adaptive insert to provide the electrical connectionbetween said respective contacting surfaces and said respective branchcurrent leads.
 12. A termination unit according to any one of claims8-11 wherein at least one of said adaptive inserts is located betweenelectrical conductor(s) of the contacting surface of the neutralconductor or electrical phase and an underlying insulation layer.
 13. Atermination unit according to any of the above claims comprising one orseveral longitudinal spacers that provides a fixed distance between eachof the branch current leads.
 14. A termination unit according to claim13, where the longitudinal spacer(s) function(s) as flow control or flowrestraint for the cooling fluid guiding at least a part of the coolingfluid through the heat exchanging means near the contacting surface orthe base of the base current lead.
 15. A termination unit according toany one of claims 13-14 where the longitudinal spacers providesubstantial equi-distance between each of the branch current leads. 16.A termination unit according to any one of claims 1-15 wherein at leastone of the neutral conductor or electrical phases has electrical contactto a current sleeve, said current sleeve optionally being at leastpartially of a conventional electrically conductive material.
 17. Atermination unit according to claim 16 where the current sleeve is atleast partly of a superconductive and/or hyper conductive material. 18.A termination unit according to any one of claims 16-17 where at least apart of the current sleeve provide the electrically connection betweensaid contacting surfaces and said respective branch current leads.
 19. Atermination unit according to claim 16-18 wherein said current sleevehas a sleeve contacting surface for being electrically connected to acurrent lead.
 20. A termination unit according to any one of claims16-19 wherein superconducting material from the cable itself constitutesat least in part said sleeve contacting surface.
 21. A termination unitaccording to any one of claims 16-20 wherein two or more of the neutralconductor or electrical phases have electrical contact to respectivecurrent sleeves, the dimensions of said current sleeves preferably beingsubstantially identical.
 22. A termination unit according to any one ofclaims 16-20 wherein a substantially equal radial distance from thecentral longitudinal axis of the sleeve contacting surface of theneutral conductor and electrical phases is provided, preferably byindividual adaptation of radial dimensions of said current sleeves. 23.A termination unit according to any one of claims 1-21 comprising atleast one current clamp made at least in part from a conventionalelectrically conductive material, (such as Cu but not limited to) saidcurrent clamp being electrically connected to a current lead, and beingclamped to said contacting surface and/or to said sleeve contactingsurface, said current clamp preferably being shorter than the axiallength of the contacting surface and/or shorter than the sleevecontacting surface.
 24. A termination unit according to any one ofclaims 1-23 wherein at least one of said electrically connections,preferably at least one of said current clamps comprises through goingholes.
 25. A termination unit according to any one of the precedingclaims wherein at least one of the electrical phases and neutralconductor comprises superconductive or hyper-conductive material.
 26. Atermination unit according to any one of the preceding claims, whereinat least one of said electrical connections comprises an electricalfield-control system totally or partly surrounding the contactingsurface and at least a part of said branch current lead.
 27. Atermination unit according to claim 26 wherein the electricalfield-control system comprises a dielectrics, such as ULTEM™ orfiber-reinforced epoxy plastic such as G10.
 28. A termination unitaccording to any one of claims 26 and 27 wherein the electricalfield-control system comprises field smoothening material e.g. a metal-or semi-conducting material, said field smoothening material preferablyconstitute a layer of the electrical field-control system, morepreferably an inner layer of the electrical field-control system.
 29. Atermination unit according to any one of claims 26-28 wherein theelectrical field-control system provides a transition joint to one ofsaid branch current lead and said branch electrical insulationinsulating said branch current lead.
 30. A termination unit according toclaim 29 wherein said transition joint is made between surfaces formingmatching zig-zag paths or meander paths to increase the creep length orsaid joint having spherical surfaces to allow for angular adjustment ofthe branch insulation system, preferably said transitional joint issealed by a gasket.
 31. A termination unit according to any of the aboveclaims wherein at least one of said branch current leads comprising acurrent lead section comprising a cold end connected to a neutralconductor or an electrical phase, an opposite ambient temperature endand an intermediate thermal anchor section, said intermediate thermalanchor section provides the a thermal gradient from cold to ambienttemperature.
 32. A termination unit according to any of the above claimswherein at least one of said branch current leads comprising a currentlead section comprising thermal anchor section that provides the optionof a thermal gradient from cold to lower temperatures.
 33. A terminationunit according to any one of the preceding claims wherein the number ofphase conductors N, is N=1.
 34. A termination unit according to any oneof the preceding claims wherein the number of phase conductors N, isN=3.
 35. A termination unit according to any one of the preceding claimsand comprising at least one of a sleeve, an insert and a clamp whereinsaid sleeve(s), insert(s) and/or clamp(s) has/have each the form of acuff.
 36. A termination unit according to claim 35 wherein the groupconsisting of the sleeves, inserts and clamps comprise two, three ormore parts facilitating mounting.
 37. A termination unit according toany one of claims 1-36 wherein the edges and/or surfaces of longitudinalspacers, adaptive inserts, sleeves, clamps, current lead bases arerounded and/or adapted to control the electrical field.
 38. Atermination unit according to any one of claims 3-37 wherein at leastone of the inserts is made of material having a specific electricalresistivity above 10̂6 ω·m²/m over the operating temperature range.
 39. Atermination unit according to any one of claims 3-38 wherein at leastone of the inserts comprises an epoxy material, such as a fibrereinforced epoxy resin, such as a thermosetting industrial laminateconsisting of a continuous filament glass cloth material with an epoxyresin binder.
 40. A termination unit according to any one of thepreceding claims wherein said terminal unit comprising a rigid cylinderor tube located in an opening at the centre of the core former of theend-section of the cable, for providing mechanical support for the endsection of the cable.
 41. A termination unit according to any one ofclaims 1-40 for providing and electrically terminating a cryogenicallycooled multi-phase cable system to termination current leads at ambienttemperature, said cable end-section being an end section of amulti-phase cable, the end-section of the multi-phase cable beingsurrounded by a thermally insulating envelope for enabling the coolingto—and the maintaining of at least a part of the end-section of thecable at a temperature below ambient, wherein the thermally insulatingenvelope comprises a plurality of branches in the form of branch currentlead for branching off from the end-section of the cable electricalphases and the neutral conductor to respective current leads, thebranches being arranged sequentially along the longitudinally extendingend-section of the cable at locations matching the location of thecontacting surfaces of the corresponding end-parts of the neutralconductor and electrical phases in question, the first branch being forbranching off the neutral conductor, the second branch for the Nthelectrical phase, the third branch for the (N−1)^(th) electrical phase,etc., and finally the (N+1)^(th) branch for branching off the N^(th)electrical phase.
 42. A termination unit according to claim 41 whereinthe thermally insulating envelope has an inner cylindrical surface, theend-section of the cable being located along the surface and the pointsof contact between said surface and said cable defining a substantiallystraight contacting line.
 43. A termination unit according to any one ofclaims 41-42 wherein at least one of the said branches is substantiallyperpendicular to the longitudinal axis of the end-section of the cable.44. A termination unit according to any one of the claims 41-43 whereinthe end-section of the cable is eccentrically located in the thermallyinsulating envelope.
 45. A termination unit according to any one ofclaims 41-44 wherein said thermally insulating envelope enclosing saidend-section of said cable is modularly constructed, each modulecomprising a part for enclosing a length of the end-section of the cableand at least one branch for branching off the neutral conductor or anelectrical phase.
 46. A termination unit according to any one of claims41-45 wherein the thermally insulating envelope comprises two separatefluid coolant flow paths.
 47. A termination unit according to any one ofclaims 41-46 wherein at least a part of said fluid coolant is restrictedto a flow path that is unique for the termination unit and does not passthrough the rest of the superconducting cable.
 48. A termination unitaccording to any one of claims 1-44 comprising a field-control system incontact with an electrically insulating material located around theend-section of the cable at branches of the thermally insulatingenvelope, the field-control system preferably defining an equi-potentialvolume at phase voltage and controlling the path of the electric fieldsin the vicinity of the branch from phase voltage to a ground potentialof the termination housing.
 49. A termination unit according to claim 48wherein the field-control system comprises, but is not restricted to, amaterial selected from the group of materials comprising Ultem™, G-10,FRP, Polyethylene, Polypropylene, Nylon and combinations thereof.